Field of the Invention
The invention relates to a magnetic-inductive flowmeter for measuring flow of a flowing medium, having a measuring tube, having a magnetic field generator for generating a magnetic field at least partially interfusing the measuring tube, having at least one measuring electrode for tapping a measuring voltage induced in the flowing medium, wherein the measuring tube has a central measuring section including the measuring electrodes that is flat on one side having a planar measuring tube portion, called portion in the following, and at least one coil core and one pole shoe belong to the magnetic field generator.
Description of Related Art
According to Faraday's law of induction, an electric field strength is formed perpendicular to the direction of flow of the medium and perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field. Faraday's law of induction is thus exploited in magnetic-inductive flowmeters in that a magnetic field usually fluctuating over time during the measurement process is generated by means of a magnetic field generator usually having at least one magnetic field coil, one coil core as well as one pole shoe and that the magnetic field at least partially interfuses the electrically conductive medium flowing through the measuring tube. Here, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
At least one measuring electrode for tapping a measuring voltage induced in a flowing medium belongs to the magnetic-inductive flowmeter being discussed here. Often, two measuring electrodes are present. Preferably, the measuring electrodes come into contact with the medium and the virtual connection line of the two measuring electrodes runs at least essentially perpendicular to the direction of the magnetic field interfusing the measuring tube perpendicular to the longitudinal axis of the measuring tube.
It has already been described above that the measuring electrodes are, in particular, such that they come into contact with the medium, i.e., tapping the measuring voltage while galvanically in contact with the medium. However, there are magnetic-inductive flowmeters in which the measuring voltage is capacitively tapped.
Magnetic-inductive flowmeters known from the prior art in German Patent DE 692 32 633 C2, German Patent Application DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754 B1, German Patent Application DE 100 64 738 B4 and corresponding U.S. Pat. No. 6,564,612 B2, German Patent Application DE 102 43 748 A1 and corresponding U.S. Pat. No. 6,804,613 B2, German Patent Application DE 10 2008 005 258 A1 and corresponding U.S. Pat. No. 7,971,493 B2 and German Patent Application DE 10 2011 112 703 A1 and corresponding U.S. Patent Application Publication 2012/0066301 A1 as well as European Patent Application EP 0 704 682 A1 and European Patent Application EP 0 834 057 A1 and corresponding U.S. Pat. No. 6,092,428 are referred to as examples. Reference is made, in particular, to German Patent Application DE 10 2008 057 756 A1 and corresponding U.S. Pat. No. 8,286,502 B2, from which the magnetic-inductive flowmeter described in the introduction is known. The measuring tube in this known magnetic-inductive flowmeter has a changing cross-section over its length and the cross-section in the central section of the measuring tube, called measuring section above, is less than at the beginning of the measuring tube and the end of the measuring tube. Thereby, the cross-section of the measuring tube in its central section, i.e., in the measuring section, is rectangular, or optionally square. On the other hand, the invention is based on a magnetic-inductive flowmeter, in which the measuring section of the measuring tube necessarily has at least one planar measuring tube portion, called portion above. The measuring section can have two or more planar portions.
The known magnetic-inductive flowmeters are often a “sturdy construction” in that the measuring tube consists of metal. As a general rule, these measuring tubes are tubes, i.e., cylindrical hollow bodies having a circular cross section. Further, it holds true for most magnetic-inductive flowmeters that the measuring device housings have end flanges and connection flanges consisting of metal on both sides. On the one hand, these flanges, with which the two ends of the measuring tube are—directly or indirectly—connected, terminate the flowmeter, leading to the term “end flange”. On the other hand, the flanges are used for connection of both sides of the flowmeter to the corresponding piping flanges, thus “connection flange”.
Magnetic-inductive flowmeters have to meet substantial requirements for measuring accuracy. It is to be taken into consideration here that the measuring voltage induced in the flowing, electrically conductive medium is relatively low. This holds true for “normal conditions”, which should be understood as a flow velocity that is not particularly low and a conductivity of the flowing medium that is not particularly low. In the case of “complicated conditions” in terms of measuring, i.e., low flow velocity or low electric conductivity of the flowing medium, particularly low measuring voltages are induced, with the consequence that relatively low—in absolute terms—interfering voltages significantly influence the measuring accuracy.
For this reason, the magnetic field generator should generate a relatively strong and largely homogenous magnetic field in the measuring tube. This takes place in particular in that the measuring tube should only be relatively thin in the measuring section.
In an implementation of the magnetic-inductive flowmeter known from German Patent Application DE 10 2008 057 756 A1 and corresponding U.S. Pat. No. 8,286,502 B2, the wall thickness of the measuring tube in the measuring section is less than at the beginning and the end of the measuring tube. In order to guarantee a certain capacity of the measuring tube to withstand pressure, even in the measuring section, at least one reinforcement connecting the measuring tube to the measuring device housing is provided in the central section of the measuring tube, i.e., in the area of the measuring section.
Instead of a measuring tube of metal, it can be appropriate to produce the measuring tube of a material that is relatively less durable—and thus less expensive—in particular, of relatively inexpensive plastic. Such a material requires that its relatively low ability to withstand stress has to be accordingly compensated. On the other hand, however, it is possible that even in the case of metallic measuring tubes, such high pressure arises that the required stability needs to be ensured.
In order to generate the above described strong and homogenous magnetic field, it is further provided to create at least one flat and planar measuring tube portion, called portion in the following, in the measuring section and to arrange the pole shoe of the magnetic field generator next to and flat against the planar portion and thus close to the inside of the measuring tube.